Immunological mechanisms have been considered to play a major role in the pathogenesis of sys- temic lupus erythematosus (SLE).Although y-globulin and complement have been demon- strated in the renal lesions of SLE (1-10), the specific immunoglobulins have not been character- ized.The presence of fibrin has been previously noted (8, 10, 11), but the relationship of glomeru- lar fibrin deposition to renal damage has not been ascertained.Therefore, the fluorescent antibody technique was employed to study the localization of the following proteins in tissue lesions: 1) im- munoglobulins (Y2-, y1M-, and ylA-globulins), 2) p1c-globulin (complement), 3) fibrinogen, and 4) other plasma proteins (a2-macroglobulin and al- bumin). Methods-Necropsy specimens of kidney, liver, spleen, and heart from 16 patients with SLE and 4 normal kidneys from patients with no evidence of renal disease were frozen in dry ice-isopentane and stored at -20°C.Clinically characteristic cases of SLE with positive LE cell tests were chosen.The pertinent clinical and pathologic fea- tures are summarized in Table I.All specimens from patients with SLE showed a moderate to severe nephri- tis with typical wire loop lesions, focal necrosis, and oc- casional hematoxylin bodies.Arteriolar fibrinoid necro- sis was present in the kidneys and spleen and occasionally in the heart and liver.Formalin fixed paraffin sections were stained with hematoxylin-eosin, Gomori's elastica stain, phosphotung- stic acid-hematoxylin (PTAH), and Lendrum's fibrin stain (12) and were subjected to the periodic acid Schiff reaction after diastase treatment.